using "that is" in a sentence

I’m a bit confused about the use of “that is” to add explanatory info about what is being talked. Also, can I also use it when the noun deserving explanation is plural? Here an example:

Encircling the town were military stations of “changers”, that is men
hired by the government to make sure that all inhabitants obeyed the
rules.

Is this sentence correct?
Also, in this case the noun that requires explanation “changers” is plural, yet I suppose I don’t have to write “that are” instead of “that is” nor do I have to use the past tense and write “that were” or “that was”.

Thank you